1. Field of the Invention
The present invention provides a process for solubilizing an organic or mineral salt. More particularly, the invention provides a process for solubilizing an organic or mineral salt in an organic solvent in which the organic or mineral salt is not soluble, or for increasing the solubility of an organic or mineral salt in an organic solvent.
2. Background of the Prior Art
The problem of solubilizing salts is a significant one. It is well known that many organic and mineral salts are either insoluble or lack satisfactory solubility in the majority of organic solvents used industrially.
Organic solvents may be classified according to their ability to dissolve a salt. This classification takes into account various factors, such as, specifically, the polar and protonic character of these solvents. It is well known to those skilled in the art that polar solvents, i.e. solvents having a high dielectric constant, dissolve salts more readily than solvents of an apolar character, i.e. those having a low dielectric constant. It is also well known that the best polar solvents are the protonic polar solvents, i.e. solvents having a high dielectric constant and possessing hydrogen atoms of an acid character.
Thus, the best organic solvents to dissolve salts include formamide, acetamide, formic acid, hexamethylphosphorotriamide (H.M.P.T.), dimethylsulfoxide, sulfolane, N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methanol and acetone, while the less good ones include chloroform, methylene chloride, carbon tetrachloride, trichloroethylene, dichloroethane, chlorobenzene, orthodichlorobenzene, benzene, toluene, the xylenes, cyclohexane and hexane.
It should be noted that the difficulty of industrial application of these solvents increases generally when proceeding from the solvents of lesser capability to the better ones. In fact, the better solvents may react with the salts to be dissolved and yield secondary products. Some of them are highly toxic, such as HMPT, while others, such as dimethylsulfoxide, are malodorous and lack thermal stability. Furthermore, all of these products are expensive. It is obvious that persons skilled in the art prefer, when possible, to use an apolar solvent such as toluene and the xylenes, for example, which are less onerous and more easily handled, because of their low toxicity and their thermal and chemical stability.
However, it is very frequently the case that a given salt may be practically totally insoluble in any of the apolar solvents and only partially soluble in the protonic polar solvents, with the maximum solubility being attained in the best of the protonic polar solvents.
It is readily seen therefore, how important it would be to be able to solubilize a salt in a solvent in which it is initially insoluble, but which solvent is easier to apply industrially, or to increase the solubility of the salt in such a solvent. This dual objective is in fact attained by the present invention, with important industrial applications being set forth hereinafter.
From the prior art, a certain amount of work leading to partial solutions in this field is known. Thus, French Patent Application No. 69.43879, published under No. 2,026,481, describes macrocyclic polyether compounds. These compounds can form complexes with the cations of certain metallic compounds, in particular with the salts of alkali metals and alkaline earth metals. These complexes are analytical reagents for use in nonhydroxylated media in which uncomplexed metal compounds are normally insoluble. The macrocyclic compounds described in said French patent application have 15 to 30 atoms in the polyether ring and are composed of 5 to 10 --O-X units, wherein X, for a particular compound, is either (a) --CHR.sub.1 -CHR.sub.2 - or (b) --CHR.sub.1 - CR.sub.3 R.sub.4 -CHR.sub.2 - wherein R.sub.1, R.sub.2, R.sub.3 and R.sub.4 are each a hydrogen atom or an alkyl radical.
It is evident that these compounds, which are most commonly called "crown ethers", have a highly sophisticated structure. It follows that the process for their preparation is a delicate operation. Without doubt, one of the major disadvantages of the use of these crown ethers is their extremely high cost, which is a reflection of their complexity and manner of preparation. Another important disadvantage is that, on a practical level, according to the patent application cited above, these compounds can only be used with alkali metal compounds and with compounds of the alkaline earth metals having atomic weights greater than 40.
Another French patent application, Patent Application No. 70.21079, published under No. 2,052,947, also describes macrobicyclic compounds capable of complexing salts and thus rendering them soluble in solvents in which they are normally insoluble. These macrobicyclic compounds, commonly called "cryptants", also have an extremely sophisticated structure, which again implies the disadvantages emphasized above with respect to the crown ethers.
Thus, a real need exists on an industrial level to have available a simple-to-apply process for solubilizing a mineral or organic salt in an organic solvent in which it is not initially soluble or for augmenting the solubility of a mineral or organic salt in an organic solvent. The work of the present applicant lead to the development of such a process.